Over the past 9 months, we have enrolled a total of 11 patients into our study. Patients were randomized in a double-blind fashion to receive either placebo or magnesium sulfate infusion. Thus far we have confirmed that patients with congestive heart failure are profoundly deficient in their intracellular magnesium concentrations. These levels are transiently normalized by infusion of 12.5 grams of intravenous magnesium sulfate. Intracellular levels are close to the initial, sub- normal levels within 48 hours, and return to the pre-infusion levels within one week. There is a temporal improvement in premature ventricular contractions, as measured by Holter monitoring, which correlates with intracellular magnesium concentrations. Currently we are analyzing the electrophysiologic parameters, measured by five minute continuous ECG monitoring, and their relationship with intracellular magnesium concentrations. Preliminary analysis demonstrate a linear relationship between coherence of QT variability and heart rate with intracellular magnesium concentrations. Further analysis of other electrophysiologic parameters is currently in progress. In addition, we have evaluated left ventricular contractility before and after study drug infusion, using a novel technique established by David Kass, M.D. This portion of the study remains blinded, however our hypothesis is that contractility will improve in subjects receiving magnesium.